The present invention relates generally to the manufacture of semiconductors, and more particularly, to an anode for plating a semiconductor wafer.
A recent trend in manufacturing semiconductors utilizes an electroplating process to deposit a metal, typically copper, onto semiconductor substrates. In a conventional electroplating process, a soluble copper anode is disposed in an electrolytic solution adjacent the substrate to be plated. The anode provides metallic ions to replenish those that are depleted during the plating process.
In such a process, it is important to produce a uniform layer of metal on the semiconductor substrate. A number of factors affect plating of the substrate. These include the uniformity of the spacing between the anode and the semiconductor wafer, the uniformity of the anode surface during dissolution of the anode and the uniformity of flow of the electrolyte between the anode and the wafer substrate to be coated.
Conventional anodes used in electroplating semiconductor substrates are usually produced as a cast ingot. Typically, these anodes have a very coarse grain structure and may include casting defects such as shrinkage pipes, voids and cracks. In addition, some copper anodes include a doping agent, such as phosphorus, to enhance performance. The doping agents in such anodes tend to be segregated within the anode structure as a result of the solidification process during casting. It has been known to mechanically roll and thermo-mechanically work the billets to provide some refinement of the grain size, but such rolling process does not always eliminate the aforementioned defects in the casting structure. In this respect, the anodes produced by casting and rolling typically have coarse grain sizes (greater than 140 xcexcm) and still contain casting defects.
The aforementioned casting defects and the segregation of the doping agent within a cast anode can produce an irregular anode surface during the electroplating process as the metal on the surface of the anode dissolves into the electrolyte. This non-uniform dissolution of the anode can interfere with the uniformity of the anode-to-wafer spacing, and can also distort the uniformity of the flow of electrolyte between the anode and wafer, both of which can adversely affect the plating of the wafer substrate.
The present invention overcomes these and other problems and provides an improved anode for electroplating semiconductor wafers.
In accordance with the present invention, there is provided an anode for use in electroplating semiconductor wafers. The anode is comprised of a metal plate formed from a metal casting that is essentially free of voids or cracks. The casting is thermo-mechanically worked until the metal of the plate has an average grain size of less than 100 xcexcm.
In accordance with another aspect of the present invention, there is provided a method of forming an anode for use in plating a semiconductor wafer, comprising the steps of:
a) casting a metal into an ingot using a semi-continuous caster; and
b) thermo-mechanically working the ingot at a temperature less than 85% of the melting temperature of the metal to reduce the cross-sectional area by at least 20% until the metal has a grain size less than 100 xcexcm.
It is an object of the present invention to provide an anode for use in electroforming semiconductor wafers.
It is another object of the present invention to provide an anode as described above that is essentially free of voids, cracks or other casting defects.
A still further object of the present invention is to provide an anode as described above that has an average grain size of less than 100 xcexcm.
A still further object of the present invention is to provide a method of forming an anode as described above.